Week 4 - Lecture 3 - Vaccination And Public Health Flashcards
Diseases where vaccines have been successful …
Diphtheria
Poliomyelitis
Meningitis C
Vaccination goal
Reduce mortality and morbidity from vaccine preventable infections
Strategic aim:
- selective protection of the vulnerable
- Elimination (herd immunity )
- eradication
Programmatic Aim
- prevent deaths
- prevent infections
- prevent transmission (secondary cases )
- prevent clinical cases
- prevent cases in a certain age group
Protecting selected high risk groups
I.e hep B, influenza , pneumococcal disease
Target high risk individuals to offer vaccination
Where risk of exposure to infection or the consequences of infection is higher in readily identifiable group
To contain or eliminate an infection in a population
I.e tetanus , measles and diphtheria
Propose is to reduce number of infections
To interrupt transmission to humans
To generate herd immunity
To prevent outbreaks and epidemics
To eradicate an infectious agent
I.e smallpox , polio
Where no other reservoirs of infection exists in animals or environment
Where consequences of infection are very high
Where scientific and political prioritisation exists
Basic concept of vaccination theory
For infections transmitted from Person to person the crucial factor determining spread of Infection is : how many secondary cases are caused by each infectious person
Basic reproductive number (Ro)
Is the average number of individuals directly infected by an infectious agent during the infectious period in a totally susceptible population
Ro is determined by basic biological features of the microorganism and the population therefore specific to both a microorganism and a population
It is proportionate to :
- length of time that the case remains
- number of contacts a case has with susceptible hosts per unit time
- chance of transmitting the infection during an encounter with a susceptible host
Ro …
Does not fluctuate in the short term
Not affected by vaccination
Is a property of the infectious agent
Ro can differ for …
Different infections in the same population e.g in UK , R0 (measles ) >Ro(rubella)
The same infection in different populations e.g for measles , Ro(Nigeria)>Ro (UK)
Effective reproductive number R
Is the actual average number of secondary cases per primary case observed in a population
R is usually smaller than Ro and reflects the impact of control measures and <100% susceptibility in a population
R …
In a homogenously mixed population , where there is the proportion susceptible ,
R=Ro*s
Susceptible populations
- any person who is not immune to a particular pathogen is said to be susceptible
- a person may be susceptible because they have never encountered the infection or the vaccine against it before
- a person may be susceptible because are unable to mount an immune response
- a person may be susceptible because vaccination is contraindicated for them
R=Ro *s
Example Ro =10
S=20% R=10 * 20% = 2.0 - an infected person will infect 2 people
s= 10% R= 10 * 10% = 1.0 - an infected person will infect 1 person
And so on …
How do mass vaccination programmes impact the disease
- reduce size of susceptible population
- reduce number of cases :
- reduce risk of infection in population
- reduce contact of susceptible to cases
- lengthening of epidemic cycle (honeymoon phase )
- increase mean age of infection
Epidemic threshold
If R>1 the number of cases incr
If R < 1 the number of cases decr
R =1 is the epidemic threshold
To achieve elimination we need to maintain R<1
